In milling certain types of materials it is often necessary or desirable to have a positive control of the atmosphere within the mill at all times. For example, readily oxidizable materials such as aluminum, titanium, magnesium, lithium and fine powders of many compositions are combustible or even explosive under certain conditions or they may be contaminated by the presence of air. In milling such materials the control of the atmosphere must extend to charging and discharging of the mill without opening the mill to air.
The present invention is not restricted to any particular materials. However, it is described below with reference to metal powders which are readily oxidized and are prepared as dispersion strengthened materials or alloys by powder metallurgy routes. Of necessity the milling of such materials must be carried out in a controlled atmosphere. The environment in the mill may be, for example, inert or may contain low levels of oxygen, hydrogen or hydrocarbons. To obtain such an atmosphere it is generally necessary to seal the mill to air.
The problems encountered in milling powders are particularly troublesome in mechanical alloying of readily oxidizable metals such as aluminum, magnesium and lithium. Mechanical alloying has been described in detail in the literature and in patents. U.S. Pat. Nos. 3,740,210, 3,816,080 and 3,837,930, for example, involve the mechanical alloying of aluminum alloys and the composite materials containing aluminum. In practice of mechanical alloying the components of the product are charged in powder form into a high energy milling device such as a ball mill where, in an environment free of or reduced in amount of free or combined oxygen, the powders are ground down to a very fine size initially, prior to particle agglomeration in the latter stages of the process. This initial grinding increases the total surface area of the metallic powders significantly. Since any freshly exposed surface of the powder is not oxidized, it is very hungry for oxygen to the extent that the powders in this condition will burn and/or might explode spontaneously if exposed to air. Thus, any port in the mill, for example, for charge or discharge of powders, is a source of potential danger from the standpoint of the quality of the product produced and/or the possibility of fire and/or an explosion. To avoid problems of explosion, burning and contamination, the mill should be emptied while maintaining positive control of the environment in the mill and throughout the entire discharging system and with minimum retention of powder in the mill.
It has been known to operate a rotary mill with a plug in an opening in the shell, the plug being replaceable with a grate during discharge. For protection of the environment during discharge, the shell is enclosed in a housing. When the milling cycle is finished the housing is opened to replace the plug with a grate, then the housing is closed for the discharge cycle. During the discharge cycle the discharge opening is rotated to the underside of the shell, thereby permitting the powder to run into the housing. The rotation for discharge of material can be repeated. This arrangement is not satisfactory. It opens the system to the atmosphere when the plug is replaced by the grate. Powder discharged from the shell tends to accumulate in the housing, thereby requiring cleaning of the housing after each run and further opening the system to air. Opening of the housing and accumulation of powder in the housing are sources of contamination of the powder discharged from the mill and to subsequent runs in the mill. A further serious problem is that when the shell rotates inside the housing the discharging powder may be in the explosion range in terms of concentration of various portions of powder discharged in any cycle. Another proposed method for discharge is by gas sweep through the mill to pick up particles and carry them to a classification system. This involves the use of a combination of devices such as drop-out chambers, cyclones, bag filters, blowers and the like. Since the powder conveyed is combustible and/or explosive, the gas sweep system poses a significant hazard. Furthermore, it is difficult to seal against infiltration of air and against leaks. It is also difficult to control the flow of powder in the discharge.
In the present system the discharge of processed material, e.g. processed to an alloy powder, is essentially gravity-dependent, the material is not aerated, it is relatively easy to keep the entire system under sealed conditions throughout the milling and discharge cycles, and the mill is discharged with minimized retention of the discharge material in the mill. Further advantages of the present discharge system are that the maintenance of the system can be achieved with minimum disturbance to the mill, and it can be done completely from outside of the mill.
The present discharge system is especially useful for mills having shells of up to about 2 to 3 feet in length, and it is possible to empty the mill substantially completely.
The discharge system of the present invention can be incorporated into existing rotary mills, permitting them to be discharged under protective conditions.